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Answer
William Nelson ECG Quiz
This is a beautiful example of ‘concealed events’.
Strip A: after four sinus beats, the P waves disappear, allowing the emergence of an escape ventricular focus after 1.76 sec.
This might simply be ‘sinus arrest’. A more romantic and attractive explanation is that there is ‘sino-atrial exit block’ and although
‘concealed’, the SA node continues to discharge, but the signal is unable to escape from the sinus node ‘island’. The measured pauses
between the P waves represent reasonable multiples of the basic sinus cycle.
Strip B: sinus bradycardia of 40/min has developed. Due to the default of the sinus node, a junctional pacemaker awakens at 52/
min to activate the heart. The dissociated P waves ‘leapfrog’ over the junctional complexes and have three fates:
1. When the interval due to the junctional discharge and the following P wave (R-P interval) is short (P waves 3 and 5), the atrial
stimulus is blocked high in the AV junction.
2. When the R-P interval is longer (P waves 1 and 4), the stimulus is able to traverse the AV junction, but it is conducted with LBBB
aberration.
3. When the R-P interval is intermediate (P waves 2 and 6) the atrial stimulus penetrates lower into the AV junction. Although it is not
transmitted to the ventricles, the signal reaches the site of the junctional pacemaker and resets it. This is an example of ‘anterograde
concealed conduction’.
